Superconductors are used both in electronic circuitry and in bulk applications, most of which are based on high-field electromagnetic coils. In 1911, Onnes discovered that the electrical resistance of mercury fell sharply at approximately 4 K and was immeasurable below this temperature. Onnes termed this the “superconducting state”. The temperature at which this phenomenon occurs is the critical temperature (Tc), and is one of the three critical properties of superconducting materials.
When certain materials enter the superconducting state by cooling below a critical temperature Tc, their electrical resistance drops to zero and they carry electrical current with no power dissipation and no heating. This phenomenon is very weak and is easily destroyed by thermal agitation. Thus, all superconductors require a cryogenic environment with most practical superconducting devices operating in liquid helium (Tc=4 K), or in liquid nitrogen (Tc=77 K).
When the temperature exceeds a critical temperature, Tc, or when the critical field, Hc, is exceeded, superconductivity is destroyed and the material then behaves normally. Superconductivity is also destroyed, even in the absence of an external magnetic field, when the current flowing through the superconductor reaches a critical value Jc.
Tc and Hc are physical characteristics of a given material or composition, but Jc is more dependent on the structural properties and history of the material. The three critical properties of a superconductor, Hc, Tc, and Jc are interdependent and create a three-dimensional space within which lossless bulk supercurrents can flow. These superconductors are referred to as (Type I) superconductors.
In practical high field (Type II) superconductors, the critical field Hc is replaced by a low critical field Hc1 and an upper critical field Hc2. In between these two critical fields the superconductor is in the so called mixed state. When the magnetic field exceeds a value of Hc1, the magnetic flux can penetrate into the bulk of the material in the form of fluxoids (individual quantum units of flux surrounded by a circulating vortex of supercurrent) until a much higher magnetic field Hc2 is reached, where the fluxoids overlap and the material become normal or resistive.
Under changing magnetic fields, the fluxoids move and generate heat. It is necessary to remove the heat generated to maintain the low temperature required for superconductivity. One technique for removing heat is to surround the superconducting material with a good thermal conductor. The combination of very small superconducting filaments (high surface area) embedded in a material of high conductivity has been used successfully to solve this problem.
As current is introduced into the superconductor, a Lorentz force (FL=J×B), perpendicular to the applied current and field acts on the fluxoids. In an ideal material with no imperfections in the field region between Hc1 and Hc2, transport currents would normally cause the fluxoids to move due to the Lorentz force, and the material would go normal resulting in a low critical current density, (Jc). However, grain boundaries, dislocations, and other imperfections can trap or pin the fluxoids and enable a high Jc to be obtained in these materials even at high magnetic fields. Such defects are therefore, referred to as pinning sites or pinning centers. Therefore, it is desirable to prevent the fluxoids from moving. These pinning sites offer an opposing force to FL known as the flux pinning force, Fp. The critical current density Jc can thus be defined as Fp=Jc×B. If the applied field or current is great enough, FL exceeds Fp and flux motion occurs.
By the 1960's it was recognized that a Nb3Sn superconductor could sustain critical current densities (Jc) exceeding 103 A/mm2 even in fields as high as 8.8 T. It was discovered that a class of superconductors with an upper critical field (Hc2) much higher than Hc1 existed. Most of the superconducting materials used in engineering applications today exhibit this type of superconductivity.
In the manufacture of practical high-field superconductors, the aim is to optimize simultaneously Hc2, Tc, Jc and the mechanical properties of the material. Most superconductors currently being manufactured are made as tape or wire. Thus, optimization must be done to material in either a tape or wire form and hence suitable for winding into coils. Wire consists of a composite of fine (<100 μm), twisted, superconducting filaments embedded in a non-superconducting matrix. This non-superconducting matrix is a material of high thermal and electrical conductivity such as copper which is typically utilized as stabilization against transients which may otherwise push the superconductor into the normal state.
To achieve a high critical current density (Jc) in a magnetic field, a superconductor must have defects or second-phase inclusions that pin the fluxoids (the vortex lattice) at the location of the defect. This microstructure can be produced in Nb47 wt % Ti, the dominant material used for commercial electromagnet applications such as Magnetic Resonance Imaging (MRI) by applying heat treatments to precipitate α-Ti out of a homogeneous Nb47 wt % Ti alloy. However, this approach limits the maximum Ti pin volume to approximately 21%. Other methods of artificially increasing the defect density have also been attempted including ion radiation and cation substitution.
A second method, artificial pinning centers (APCs), has been used to introduce pins in Nb47 wt % Ti wires. Artificial pins are placed in the Nb47 wt % Ti at a macroscopic size after which the composite wire is repeatedly drawn to produce nanometer pin thickness and spacing. The artificial pin materials used have been either Nb (low field superconductors), Cu or Ti (normal state metals). The optimum pin volume has been between 10% and 30%. APC composites can approximate ideal flux-pinning structures in a controlled design approach, which makes them valuable for all superconductors.
Another important example of Type II superconductors is Nb3Sn. This superconductor is a brittle intermetallic compound. Like Nb47 wt % Ti, this superconductor is embedded in a normal conducting matrix for electrical and thermal stability. These stability considerations further require the Nb3Sn filaments to be distributed as very fine filaments that are preferably smaller than 50 μm. Because Nb3Sn is a brittle intermetallic, Nb and Sn components are assembled with copper into a composite, then extruded and drawn into a wire while the composite is in a ductile state. The formation of the Nb3Sn superconductor is achieved when the wire is at final size. The formation of Nb3Sn occurs through a solid-state diffusion reaction at high temperature (about 600° C. to 800° C.) in an inert atmosphere. During the reaction Sn diffuses into the Nb filaments and forms Nb3Sn.
After the reaction to form Nb3Sn is completed, the matrix surrounding the filaments still contains a significant amount of Sn and, therefore, has a relatively high resistance. This area, that is the filaments and matrix together with that of the diffusion barrier, is generally referred to as the non-copper area and is the area which is used to calculate the current densities. The diffusion barrier separates the non-copper area from the copper stabilizer needed for good electrical and thermal stability.
Generally, there are three large scale processes to fabricate LTS Nb3Sn wire. The first approach is the so called “bronze process”. In this approach the Nb filaments are embedded in a bronze matrix that includes about 13 wt % Sn. The Nb filaments and bronze matrix are typically separated from the outer copper stabilizer by a diffusion barrier. This method requires intermediate anneals and is disfavored for uses requiring higher superconducting Jc because of low amounts of supercurrent delivered at high magnetic fields compared to the second approach, the so called “internal-tin” or “external-tin process”.
In the internal-tin approach Sn cores are surrounded by Nb filaments embedded in a copper matrix. The entire sub-element or sub-elements if more than one Sn core is involved is again surrounded by a diffusion barrier with the copper stabilizer on the outside. In the case of the external-tin approach, the Sn cores are located outside the bundle of copper clad Nb filaments. A diffusion barrier surrounding the Cu, Nb, and Sn components is also included as utilized in the internal-tin approach.
The third approach is the “powder-in-tube” (PIT) process. In the PIT process a powder such as NbSn2 containing 72 wt % Sn is inserted in Nb tubes and these Nb rods are then stacked in a Cu stabilizer matrix. This method, which is described in U.S. Pat. No. 5,043,320 to Meyer, et al is incorporated by reference herein.
There remains a need for superconductors with high Jc values and design flexibility which can be economically produced in bulk and which are suitable for different superconducting applications. There is also a need in the superconductor community to improve the cost-effectiveness and design flexibility of Nb3Sn, NbTi and other superconductors. The present invention method improves the general qualities and characteristics of superconductors.